Many forms of prior art capacitance measurement systems utilize a capacitive bridge configuration to produce a signal representative of the relative capacitances associated with sensor and reference capacitors in the bridge. Other forms in the prior art include systems exemplified by U.S. Pat. No. 3,048,775 wherein a capacitive sensor is configured in the feedback path around an A.C. amplifier driven by a reference A.C. signal, and wherein the amplifier output is demodulated using a diode detector.
While such prior art capacitance measurement systems are suited for certain uses as accelerometers, scales and balances, proximity detectors, linear and angular position sensors, and pressure transducers, the prior art systems are substantially limited in accuracy and sensitivity of their respective measurements due to a number of factors, including temperature variation, stray or leakage capacitance, non-linearities in the transducer configurations, noise, and drift. Furthermore, many prior art systems are limited to certain operational frequencies.
Another prior art form is described and claimed in U.S. Pat. No. 4,054,833, entitled CAPACITANCE MEASURING SYSTEM, which is hereby incorporated by reference, and is referred to as the '833 patent herein. In the '833 patent, a sensor network, including series connected capacitors, is utilized with a switching network in a feedback configuration to generate the signal which is a predetermined function of the capacitance of the sensor capacitor. In one form disclosed in the '833 patent, the feedback signal is a linear function of the reciprocal of the capacitance of the sensor capacitor. In another form, the feedback signal is a linear function of the capacitance of the sensor capacitor.
The switching network includes a two state switch and a control for cyclically controlling the switch to flip between its two states. A means is provided to couple a plurality of reference potentials to various terminals of the switch, to the feedback network, and in some embodiments to the sensor network.
The feedback network is coupled to both the sensor and switching networks, and provides a feedback signal representative to the difference in charge stored on the series capacitors from a predetermined value.
The switching network is operative to cyclically interconnect the feedback network and sensor network with the feedback signal and external reference potentials in a manner whereby the average charge stored on the series capacitors over each cycle is substantially equal to the predetermined value.
In one form disclosed in the '833 patent, the feedback network includes a differential amplifier coupled to the junction of the series capacitors in a null-seeking configuration. Additionally, the amplifier output may be directed by way of a demodulator to provide the feedback signal. The demodulator in such embodiments may be a synchronous demodulator, operating synchronously with the switch control in the switching network.
In null-seeking configurations such as the one described in FIG. 8 and FIG. 9 of the '833 patent, the feedback nulling of the error signal at the junction between the sensor and reference capacitors virtually eliminates the effect of any leakage capacitance which might be present. As a result, such configurations may readily be utilized with a remote sensor having one terminal at system ground potential.
However, in order to implement the null-seeking configuration with a remote sensor having one terminal at system ground potential, the configuration disclosed in the '833 patent decouples the power supply rails of the amplifier network from the primary system voltage and ground references. Although the potential difference across the amplifier network supply terminals remains substantially constant, the voltages at each such terminal may be offset at any given time with respect to the system ground potential. Since the components used to accomplish this decoupling are relatively large in physical size, the null-seeking technique described in the '833 patent is not conducive to implementation on an integrated circuit.
It is an object of the present invention to provide an improved capacitance measuring system wherein a signal is generated which is a predetermined function of the capacitance of a sensor capacitor, where the sensing capacitor is coupled to system ground potential.
A further object is to provide an improved capacitance measuring system which is relatively insensitive to temperature variation, stray or leakage capacitance, noise, drift and operational frequency, and which is amenable to integrated circuit implementation.
Other objects of the present invention will in part be evident and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements, and arrangement of parts exemplified in the following detailed disclosure, and the scope of the application of which is indicated in the claims.